1. Field of the Invention
This invention relates to a speed control system for a working vehicle including a stepless transmission, e.g. a hydrostatic stepless transmission, in a propelling system, the transmission being operable to provide varied speeds by pivotal movement of a shift lever.
2. Description of the Related Art
A conventional speed control system as noted above is disclosed in U.S. Pat. No. 4,341,129 or Japanese Patent Publication Kokai No. 2-102832, for example. In the conventional system, a hydrostatic stepless transmission has a speed control shaft urged to return to a neutral position by a neutralizing mechanism. The neutralizing mechanism includes a control arm fixed to the speed control shaft to be rotatable in unison therewith and defining a V-shaped recess to act as a cam. A spring-loaded cam follower is pressed on the cam to urge the control arm, and thus the speed control shaft, to return to the neutral position.
In the conventional structure, the neutralizing cam is directly connected to the speed control shaft as noted above. Consequently, where the neutral position is deviated due to an assembly error, an operation to adjust the neutral position is difficult.
Further, efforts have been made in recent years to improve driving environment by providing not only a vibration damping structure for the engine but also a damping support for mounting the hydrostatic stepless transmission on the vehicle body frame. However, in spite of the damping support for the hydrostatic stepless transmission, vibrations are transmitted from the speed control shaft through an interlocking mechanism connecting the speed control shaft to the shift lever. It is important to prevent this. For this purpose, a damping element may be mounted in an intermediate position of the interlocking mechanism. A maximum damping effect will be produced with the speed control shaft. However, since this shaft has the neutralizing mechanism requiring precision, some other position must be selected for incorporating the damping element. Thus, a sufficient damping effect cannot be expected.